The present invention relates to papermakers fabrics and, in particular, fabrics intended to facilitate the initial formation of an aqueous paper web in the manufacture of paper.
Papermaking machines generally are comprised of three sections: forming, press, and drying. Papermakers fabrics are employed to transport a continuous paper sheet through the papermaking equipment as it is being manufactured. The direction of travel of the fabric on the papermaking equipment is commonly referred to as the machine direction. The requirements and desirable characteristics of papermakers fabrics vary in accordance with the particular section of the machine where the respective fabrics are utilized.
In particular, in the forming section of papermaking equipment, forming fabrics are utilized to initially create an aqueous paper sheet or web from a pulp slurry. Typically, the pulp slurry is deposited on the top or paper carrying side of the moving forming fabric which transports the slurry over suction boxes or other means to form the paper web. The surface characteristics and drainage characteristics of the forming fabric play an important role in the initial formation of the aqueous paper web.
Multilayer forming fabrics are known in the art. For example, U.S. Pat. No. 5,025,839 discloses a dual layer forming fabric for use in the papermaking process which includes single system of zig zagging machine direction (MD) yarns which interweave with two layers of cross machine direction (CMD) yarns.
U.S. Pat. Nos. 5,013,330; 4,515,853; and 3,885,603 disclose forming and other types of papermakers fabrics having distinct top and bottom interwoven fabric layers which are interconnected by binding or stitching yarns. Such fabrics are generally known as "triple layer" fabrics, the "third" layer being the layer of binding or stitching yarns which interconnects the upper yarn system with the lower yarn system.
Early triple layer fabrics utilized different size machine direction (MD) yarns in the upper and lower fabric layers; the upper MD yarns being roughly one half the diameter than the lower MD yarns with twice as many upper MD yarns than lower MD yarns. Common weave patterns for the bottom or machine side layer were plain weave and 1:3 broken twill.
One of the problems with such triple layer fabrics was the relatively high caliper of the fabric. The high caliper allowed water drained from the aqueous paper web to be carried inside the fabric and caused problems of rewetting of the sheet during forming. Another draw back was that the bottom MD yarns tended to arrange themselves between the top layer MD yarns restricting straight through drainage and the ability to easily clean the fabrics. Also if the bottom MD yarns were not evenly spaced and correctly positioned they could cause "shadow marking" in the paper web formed on the fabric. Non-uniform spacing caused non-uniform drainage which attributed to undesired marking.
To decrease the caliper of triple layer fabrics, the same size MD yarns began to be used in both the top and bottom layers of such fabrics. A draw back to this construction was that in order to maintain the fine mesh top layer, much smaller lower MD yarns were employed which required comparably smaller lower CMD yarns which tended to shorten fabric life. To permit the use of larger CMD bottom yarns for better wear, it was possible to weave the lower MD yarns in pairs which, however, resulted in the reduction of straight through drainage in the fabric since the paired lower MD yarns would block the spaces between the upper MD yarns. It would be desirable to provide a triple layer fabric which permits the use of uniformly sized upper and lower MD yarns in conjunction with larger lower CMD yarns without substantially impairing straight through drainage of the fabric.